The endogenous viruses of guinea pigs include the type-B GPV and the intracisternal A particle found predominantly in cells of the transplantable L2C leukemia. It has recently been possible to isolate a clone of guinea pig cells which spontaneously produces substantial quantities of GPV. This in turn has permitted us to initiate an analysis of the size and number of viral proteins. We are now involved in producing large quantities of virus, purifying the proteins by gel chromatography, and preparing specific antisera. We have also demonstrated that the type-B GPV is present in the plasma of guinea pigs in the terminal stages of L2C leukemia, although it is not known how or where this induction occurs.